Dyrkla is a protein kinase implicated in the pathogenesis of Down syndrome (DS). 50% of children with DS has congenital heart defects. Due to its high prevalence, DS is a major cause of congenital heart disease affecting millions of children worldwide. DS is the result of complete or partial triplication of a human chromosome 21 (Trisomy 21). The characteristics of the disease include mental retardation, specific facial features and congenital heart disease. Despite intensive investigation, the molecular mechanisms leading to the development of these clinical features are poorly understood. A promising candidate is Dyrkla, a gene often present in triplicate in DS patients suffering from cardiac defects. I propose to study the role of Dyrkla in normal heart development, and, by manipulating Drykla gene dosage in mouse models of DS, to examine its role in heart malformations seen in DS. My long-term goal is to define the molecular basis for heart defects in DS children and use that knowledge to find new therapeutic targets for DS patients. The search for genes implicated in the pathogenesis of DS has led to the concept of a DSCR (Down Syndrome Critical Region). The DSCR is the smallest region triplicated in all established cases of DS. Recently two genes, DSCR1 and Dyrkla, within this critical region of chromosome 21 were shown to synergize to reduce the nuclear occupancy of the NFATc proteins. DSCR1 encodes a suppressor of calcineurin activity and NFATc nuclear import. Dyrkla is a kinase that promotes NFATc nuclear export. NFATc proteins are both transcription factors and signaling proteins. Remarkably, the phenotypes of NFATd, c2, c3 and c4 mutant mice reproduce severe forms of most of the phenotypic features observed in trisomy 21. Studies indicate that feedback loops within the calcineurin/NFAT pathway produce extraordinary sensitivity to a 1.5-fold increase of Drykla and DSCR1, reducing nuclear occupancy of the NFATc proteins and presumably leading to phenotypic features of DS. My objective is to test this model of the pathogenesis of congenital heart defects in DS. The specific aims are: Aim 1: Define the function of Dyrkla during normal heart development Aim 2: Define the perturbation of NFAT signaling in the absence of Dyrkla Aim 3: Determine if genetically normalizing the dose of Dyrkla rescues the features of DS in mouse models of Down syndrome